1. Field of the Invention
The present invention relates to discharge lamps, and in particular to the electrical control and construction of such lamps with a view to obtaining desired emission wavelength characteristics.
2. Description of the Related Art
A widely used lamp in interior lighting, the fluorescent tube, exploits the properties of a low-pressure discharge in mercury vapour (typically 7.times.10.sup.-3 torr, corresponding to a wall temperature of about 40.degree. C.) and argon gas (typically 3 torr) produced by the application of a mains, or higher, frequency alternating high voltage to a pair of either cold or heated electrodes at either end of a sealed glass tube. Such a plasma emits a number of discrete mercury emission lines by far the strongest of which is the 254 nm resonance line (up to 60% of the total lamp input power can appear in this line). The intense 254 nm UV radiation is converted to useful broadband visible radiation by a coating of red, green and blue phosphors on the inside walls of the glass envelope.
A major known disadvantage of the fluorescent lamp is the large difference in energy (inversely proportional to the wavelength of the radiation) between the exciting radiation at 254 nm and the range of visible wavelengths from 400 nm to 700 nm, i.e. there is a very large "Stokes shift". In theory the 254 nm photon has sufficient energy to produce two visible photons, e.g. two at greater than 508 nm, and a process that achieved this would bring about a major advance in overall lamp efficiency. A practical process to achieve this has to date been neither implemented nor described in principle. As a consequence a large proportion (typically 75%) of the energy delivered to a standard design fluorescent lamp is wasted as heat.
Recently it has been demonstrated that the colour of the emission from mercury/rare-gas discharges can be altered significantly by replacing the standard alternating (sinusoidal) power supply with a pulsed power supply. M. Aono, R. Itatani et al. (J. Light & Visual Environment, vol. 3, no. 1, p. 1-9, 1989) demonstrated that the relative intensity of emissions from the rare gas itself, usually insignificant, could be greatly enhanced by pulsed excitation. This was exploited to produce lamps whose phosphor emissions changed colour according to whether alternating (sinewave) or pulsed electrical excitation was used. Hitachi have demonstrated electrical control of the colour of the emission from mercury/rare-gas lamps and from xenon lamps; see for instance JP-A-5-135744 (Shinkishi et al).
The effect has been exploited to meet particular commercial requirements. For instance, OSRAM Sylvania have described (EP-A2-700074) the pulsed excitation of a neon discharge to produce a lamp suitable both as a flashing indicator light and as a brake light for automobiles.
Matsushita have reported (JP-A-7-272672) a fluorescent lamp driven by an alternating high frequency supply supplemented with a pulsed power supply. The advantage cited was an increase in the radiant intensity of the 254 nm emission and an increase in fluorescent lamp efficiency.
EP-A1-334356 (VEB NARVA) also discusses the use of pulsed discharges to produce a desired spectral emission, though here the emphasis is on the use of high-pressure caesium and/or rubidium discharges, with possible additives, and phosphors are not used.